I. Field of the Invention
This invention relates to a novel photochromic compound and to a novel photochromic polymer containing the photochromic compound, as well as to a process of producing the polymer and to a use of the polymer.
II. Description of the Prior Art
As the compounds which exhibit photochromism, spiro series compounds are best studied because of their great color change. Among these, spiropyran-based compounds show great color change, so that they are currently used by being dispersed in a solution or in a macromolecular medium, or by being chemically bonded to a carrier. It is known that in cases in which the photochromic compound is polymerized via chemical bonds, the elution of the photochromic compound from the macromolecular medium is reduced and the thermal stability of the photochromic compound is enhanced (G. Smets, "J. Polym. Sci. Polym. Chem. Ed." Vol. 12, pp.2511-2523 (1974)). There are various methods by which the spiropyran-based compounds may be carried on a macromolecular carrier. Among these, the most practical one is the method in which a polymerizable functional group is introduced into the photochromic compound, and then the compound is homopolymerized or copolymerized with a polymerizable compound. The spiropyran compounds having a polymerizable functional group are disclosed in "J. Polym. Sci. Polym. Chem. Ed." Vol. 12, pp.2511-2523 (1974), Japanese Patent Publication (Kokoku) No. 307114/79 and Japanese Patent Disclosure (Kokai) No. 76490/86.
However, the spiropyran-based compounds have a drawback in that their durability against repeated coloring-decoloring cycles is low, which hinders the practical use of the compounds.
As photochromic compounds which show large color change and have great durability against repeated coloring-decoloring cycles, indolinospironaphthooxazine is disclosed in Japanese Patent Publication Nos. 28892/70 and 48631/74, and piperidinospironaphthooxazine is disclosed in Japanese Patent Disclosure (Kokai) No. 145089/87. These spirooxazine compounds are used by being dispersed in a solution or in a macromolecular medium.
The dispersion of the spirooxazine compounds employed in the prior art has the following problems:
(1) The solubility of the spirooxazine compound is limited, so that the amount of the spirooxazine compound which may be added to the macromolecular medium is limited accordingly. Thus, a problem of low color density is brought about.
(2) When the spirooxazine compound-containing macromolecular medium is used as a coating composition, the spirooxazine compound may be precipitated because of the evaporation of the solvent.
(3) If the spirooxazine compound-containing macromolecular medium is subjected to heat, to a chemical or to vacuum, evaporation, extraction or elution of the compound may occur.
(4) The thermal and chemical properties of the spirooxazine compound-containing macromolecular medium is reduced due to the plasticization of the macromolecules.
In view of the ease of handling and chemical resistance, it is preferred that the photochromic compound be made into a polymer by homopolymerizing or copolymerizing the photochromic compound. Usually, such a photochromic polymer is used by being applied on a substrate after being dissolved in a solvent. As the solvent, an organic solvent is usually used. However, if the solution is applied on a plastic substrate, the organic solvent may dissolve the plastic substrate. Further, it is dangerous to use an organic solvent. Therefore, an aqueous solvent is preferred to an organic solvent. In view of this, a technique in which a water-soluble photochromic compound is dispersed in a water-soluble polymer is proposed (Japanese Patent Disclosure (Kokai) No. 23787/73).
However, the durability of such a composition is low. A water-soluble photochromic compound as well as a water-soluble copolymer thereof, which excells in durability, have not yet been obtained.